Method of etching minute bulbular openings in a metal sheet



May 14, 1968 M. KOCSUTA 3,333,254

METHOD OF ETCHING MINUTE BULBULAR OPENINGS IN A METAL SHEET Filed Nov. 16, 1964 H v Fig. 2 Fig. 3

Fig; 4

INVENTOR.

MICHAEL KOCSUTA BY HIS ATTO EY United States Patent 3,383,254 METHOD OF ETCHING MINUTE BULBULAR OPENING IN A METAL SHEET Michael Kocsuta, Willowick, Ohio (P.O. Box 5933, Cleveland, Ohio 44101) Filed Nov. 16, 1964, Ser. No. 411,240 4 Claims. (Cl. 156-5) This invention is related to a process of making minute, bulbular openings in a metal plate. More particularly, it is concerned with the process of making such openings of controlled shape with accurate dimensions and smooth surfaces.

For various purposes, such as in the manufacture of particular types of nuclear batteries, it is desirable to prepare metal plates having minute bulbular openings therein, accurately spaced from each other and of accurate dimensions and design. It is also important that the surface of such openings be smooth and free of craters, rough surfaces, and jagged edges. The size, number and shape of these openings make it very time consuming and practically impossible to fabricate by mechanical means.

Attempts to produce such openings by the common etching procedures have given unsatisfactory results due to the fact that the etching solution leaves rough surfaces in the neck and inner surfaces of such openings. Also, the etching solution gives very inaccurate reproduction of the desired dimension of the neck of the opening, resulting in jagged edges as well as in enlarged openings. Moreover, by such methods, the shapes of the bulbular cavities are difficult to control and the shape is very erratic and unpredictable as to dimension, quality and uniformity.

In accordance with the present invention, it is now possible to produce such openings of the desired accuracy, uniformity, smoothness and controlled shape by etching a metal plate coated with a well adhered resist coating bearing the desired design and pattern, by having the plate submerged in the etching solution at an angle of about 2 to relative to the horizontal plane, with the lower surface of the plate being completely submerged and being that side of the plate which is to be etched, having a stream of air bubbles rising through the etching solution and into contact with said plate surface, the size of the air bubbles preferably being no larger than about /2 inch in dameter, advantageously less than 4 inch in diameter, the area in which said air bubbles rise being of such dimension that the bubbles come into contact with the complete area of the plate on which the desired design is to be etched.

The angle at which the plate is maintained during the etching operation is reversed periodically so that, if this angle initially faces to the right from the bottom edge of the plate, it is reversed after a certain period so that the angle faces to the left from the opposite edge of the plate. In some cases, such a reversal need be performed only once during the etching operation. In other cases, it may be desirable to alternate these angles several times, generally increasing the angle the second or third time. In cases where the cavity is to be slanted in one direction or the other from the vertical center line of the cavity opening, the length of time during which the angle will face to the right or left will be correspondingly longer to effect such a slanted opening. In most cases, however, symmetrical openings are desired and the periods for which the angles are maintained to the right or left are approximately equal.

For most purposes it is generally satisfactory to alternate the aforesaid angle to the right and to the left. This gives a slightly oblong or oval character to the horizontal cross section of the bulbular opening at its largest Width,

as shown in FIG. 5. Where it is desirable to have this cross-section more truly circular, as in FIG. 4, this can be effected by alternating the aforesaid angle in at least four directions, namely right, left, frontward, and backward, using the corresponding different edges of the plate at the base of the angle. These directions might be designated also as north, south, east and west, or 0, 180, 270, etc. It generally is prefered that the second direction be the direct opposite of the first direction, followed by one or the other of the remaining directions.

The figures in the accompanying drawing illustrate various aspects of the invention.

FIG. 1 illustrates a typical cavity in vertical cross section having its dimension short in depth and great in width, with a narrow surface Opening 1, neck wall 2, and with bulbular opening having its greatest width or greatest horizontal cross section at 3. The metal surface is shown as 1'.

FIG. 2 illustrates a typical cavity in vertical cross section having a longer dimension in depth than in width.

FIG. 3 illustrates a cavity in vertical cross section which is slanted to the left from the vertical.

FIGS. 4 and 5 show a top view of the two cavities with the dotted lines showing the bulbular opening in its largest width. In the first case the bulbular section is circular, and in the other case, it is somewhat oblong or oval.

FIG. 6 illustrates a plate surface having a number of minute cavities etched therein. The figure shows the necks or tops of the openings, in which, in this case, each opening is approximately equidistant from adjacent openings. The first row of such openings also shows, by dotted line, the inner wall of the bulbular cavity which has a greater dimension in width than the neck of the cavity. The four corners of the etched surface of the plate are designated as 4, 5, 6, and 7, in clockwise order.

FIG. 7 shows a plan view of a plate in a mordant or etching 'bath. The plate 8 rests on supports 9 and 9' in such a manner that the angle faces to the right. When the position of the plate is alternated so that it rests on supports 9" and 9", the angle faces to the left. The mordant solution 10 is in container 11. The air bubbles 12 coming from openings in the manifold 13 rise through the solution and strike the under surface of plate 8 moving into and out of the openings as described herein, causing the desired circulation of mordant or etching solution.

Air is applied to the manifold through pipe 14 from an outside source. The manifold and pipe are made of a material resistant to the mordant solution, such as polyethylene or other suitable plastic.

Obviously, instead of alternating the angle to the right and left, or in the various directions as described above, it is also possible to obtain the same effect by rotating the plate about an axis perpendicular to the plane of the surface of the plate. For example, in FIG. 7, corner 4 of the surface of the plate shown in FIG. 6 is resting on the lower support 9 with corner 4 being on the side closer to the viewer. By rotating the plate around the aforesaid axis, corner 6 would be resting at the position formerly occupied by corner 4. In this way the same effect will be obtained as though the angle had been changed from the right to the left without rotation of the plate. Likewise, if the angle is to be alternated in four directions, the same effect can be accomplished by having the position initially occupied by corner 4, alternately occupied by corners 6, 5 and 7, respectively.

Generally it is preferred that at the beginning of an etching operation the above-described angle be approximately 2 until the opening has been etched away at least to some small extent. Then, the angle can be increased up to about 10 depending upon the width of the bulbular opening desired. This initial small angle is desired to slow down the passage of the bubbles 'along the under surface of the plate until after the neck of the opening has been formed, then after the opening has been developed to some extent, the angle can be increased and the bubbles then will effect the desired agitation and movement of etching solution in the opening.

For many purposes 'a neck opening of approximately 0.02" in diameter is desirable, or advantageously in the range of about 00006 to about 0.030". The depth of the bul'bular opening can be from about 0.006 to about 0.017" and can be controlled by the period of exposure to the etching solution and to some extent by the angle of inclination. The shape of the bulbular opening is controlled by the angle of inclination of the plate, wider and shallower bulbular openings being favored by a greater angle, and deeper and narrower openings being favored by a smaller angle.

In preparing a plate for etching in accordance with the process of this invention, any method of applying a resist coating, "as well as the desired design, and any type of resist coating can be applied provided that the resist coat ing is well-adhered and hardened to the metal surface and sufficiently resistant to the mordant or etching solution so that the design will be reproduced accurately in the metal. While photoresist materials are preferred, various other resist coatings can be used and the design imprinted or otherwise implanted on the metal surf-ace with accuracy in dimension and spacing in such a manner that the desired cavities will be produced in the metal plate by the process of this invention. While standard methods of applying resist coatings and effecting the de sired design therein can be used, particularly suitable methods of applying the resist coating and types of resist coatings are illustrated in copending applications, Ser. Nos. 19,484 and 19,485, filed Apr. 4, 1960, a typical example of which is described below in Example 1.

Various types of metals can be present in the plates in which the aforesaid designs are to be produced. The process is particularly appropriate for iron-containing plates, such as carbon steels, low alloys steels, stainless steels as well as non-ferrous alloys, such as illustrated in Langes Handbook of Chemistry, ninth edition (1956), on pages 862-887. The metals which can be present in such plates include iron, nickel, chromium, copper, and aluminum in predominant proportions and can contain minor proportions of various metals including tin, zinc, magnesium, manganese, etc. The plate can be prehardened or otherwise pretreated without interfering with applicants process.

The mordant or etching solution can be any type suitable for the particular metal being used, and the resist coating material should be one that is not adversely affected by the particular mordant solution selected.

This invention is particularly useful in the production of cavities having neck openings of approximately 0.006- 0.030 inch. There is no limitation on the number of such openings, the spacing between openings, and other designs that also may be located on the plate. While fiat plates obviously are more suited to the process of this invention, it is possible also to produce such cavities in cylindrical surfaces by obvious modifications involving very slow or intermittent rotation of the cylindrical surface in the mordant bath.

It is desirable that the size of the air bubbles be sufficiently small that they rise individually and do not unit to form large globs of air. Generally very small bubbles are preferred over the larger sizes indicated above. Obviously various adjustments in pressure, size of openings, and depth of liquid can be made to give desired sizes of bubbles. This individualizing and contact of the bubbles with the metal surface is believed to cause a movement of the etching solution within the cavities as they are being formed, thereby increasing the efiiciency of the etching solution as it acts on the metal and also distributing it more effectively over the surface of the metal so as to produce smooth edges, thereby avoiding jagged edges and craters which otherwise result.

While air is obviously more economical and most easily available other gases can be substituted for air, since the chief function of the bubbles is to effect circulation of the solution in the openings. Therefore, any gas which will not interfere with the etching operation or produce other undesirable side effects can be used.

It also is important that the resist coating be well adhered to the metal so as to prevent the mordant from working its way under the resist, thereby eating away metal and leaving a rough surface at the neck of the cavity, as well as an irregular dimension in the surface opening.

The invention is illustrated best by the following examples which are given merely to illustrate methods of practicing the invention and are not intended in any way to limit the scope of the invention. In these examples and elsewhere in the specification, parts and percentages are intended to mean parts and percentages by weight unless otherwise indicated.

Example I A metal plate on which a desired design is to be etched in accordance with the procedures described herein, is placed on a heated steel plate of -34 inch thickness and having sufficient area to support the entire metal plate. This heater plate has heater elements underneath, thermocouple for indicating temperature thereof, and controls for adjusting the temperature, and advantageously is placed in an area having good ventilation. The metal plate is heated in such a manner that the temperature throughout is relatively uniform and not overor underheated in any area. Means for measuring the temperature of the metal plate are attached or placed thereon. With the temperature of the metal plate maintained at about 125 F., the entire area of the metal plate is treated with an aqueous hydrochloric acid solution containing four fiuid ounces of concentrated hydrochloric acid (36% by weight) per gallon of solution. The solution is of such a temperature that its contact with the metal plate does not lower the temperature thereof and the metal plate is maintained at about 125 F. The acid solution is left in contact with the metal plate for about four minutes, after which the solution is washed off with water at a temperature of about 125 F.

An albumen photoresist solution (prepared according to the procedure described at the end of this example) is applied to the metal plate while the temperature is maintained at about 125 F. This resist is sprayed on the plate to give a thin, continuous, uniform coating thereon. After a coating of relatively uniform thickness has been applied, the coating is allowed to dry. The drying is completed within a matter of minutes. The plate then is removed from the heater plate and placed in an area protected against actinic light until the plate has reached room temperature. The remaining surfaces of the plate, including the sides and back, are coated with a considerable thickness of asphaltum and dried. A photographic negative bearing the image desired for reproduction is placed on the coated plate and the plate then is placed in a vacuum frame for exposure. The negative with the plate underneath then is exposed to actinic light from a 20 ampere carbon arc lamp for 6 minutes. Then the plate is taken out of the vacuum frame and developed with a calcium chloride developer (made by adding 53 ml. of lactic acid to a liter of 404l Baum aqueous calcium chloride solution) until a clear image is obtained; the plate next is washed with alcohol and dried.

The following procedure is used in preparing the albumen photoresist solution which is used in the above procedure. Egg albumen crystals (4.5 avoirdupois ounces) are placed in a cheesecloth bag and suspended in 22 ounces of water until dissolved. The bag then is removed without squcezing. A volume of this solution is measured out which contains '3 avoirdupois ounces of albumen. This can be checked by determining the density of the solution and referring to the density chart on page 49 of the book entitled Photography and Plate Making for Photolithography by I. H. Sayer, published in 1939 by Lithographic Textbook Publishing Company, Chicago, 111. To the quantity of albumen solution containing 3 counces of albumen, are added 5 liquid ounces of dichromate solution made as follows: 16 avoirdupois ounces of ammonium dichromate are dissolved in 64 fluid ounces of water, the solution is filtered and water is added slowly until the hydrometer reading indicates that the solution has reached a density of 142 Baum. A small amount of Rhoduline Blue Dye is added to give color contrast upon development of the plate.

Example II A plate of 1020 carbon steel, one inch thick, 9 inches wide, and 20 inches long, is processed according to the procedure of Example I using a negative bearing a design as shown in FIG. 6. After the resist layer has been developed, washed, and dried with the desired opening areas of the metal plate exposed, the plate is inserted in the bath shown in FIG. 7 with the image-bearing resist layer side of the plate resting downward in the position shown in FIG. 7. The angle of the plate initially is set at 2. The modant solution comprises 41 Baum ferric chloride solution agitated at 80 F. The manifold is made of polyethylene pipe having openings therein of about 0.030 inch in diameter, spaced 2 inches apart. Air is fed to the manifold at an average pressure of about psi. The manifold is located near the bottom of the tank at a depth of about 9 inches from the top surface of the solution. The plate is positioned at its highest point about 2 inches below the surface of the liquid and is exposed in the initial postion for minutes. Then the angle is reversed so that it faces to the opposite direction. After another 15 minutes of exposure in this position, the plate again is rearranged in the original direction but the angle is increased to 5. After exposure for another 15 minutes, this angle is reversed in the opposite direction at 5 for another 15 minutes. Thereafter, the plate is removed from the mordant bath and washed with water to remove the resist layer. The plate has a number of openings as shown in FIG. 6. The bulbular openings have a configuration somewhat similar to that shown in FIGS. 2 and 5. The edges and surfaces of the openings are smooth, and the neck opening is an accurate reproduction of the dimensions of the original design being about 0.020 inch in diameter.

Example III The procedure of Example II is repeated, except that the plate is made of 430 stainless steel, the exposure period each time at 2 is minutes, and the plate is alternated in the four directions, right, left, frontward, and backward, with the second angle being increased to 10 for an additional 20 minutes in each of the four directions, and the exposure at 10 is repeated in the alternating four directions for another 20 minutes in each case. The total exposure time in the various angles and various directions is thus four hours. The configuration of the bulbular openings is somewhat similar to that shown in FIGS. 1 and 4. The edges and surfaces of the openings are smooth, and the neck opening is an accurate reproduction of the dimensions of the original design being about 0.020 inch in diameter.

Example IV The procedure of Example II is repeated except that the initial angle position is maintained for 15 minutes and instead of alternating the angle direction, the angle position merely is increased to 10 for 45 minutes. The resultant cavities have a configuration resembling that of FIG. 3 with smooth surfaces and an accurate reproduction of the neck opening.

6 Example V The procedure of Example 11 is repeated with similar results using aluminum in place of the carbon steel used in Example II and using 1 minute exposures at 2 and 4 minute exposures at 5".

Example VI The procedure of Example II is repeated with similar results using copper instead of carbon steel, a shellacammonium dichromate photoresist in place of the resist used in Example II, and exposure periods of 5 minutes each in the two directions at 2 and 10 minutes each in the two directions at 5.

Example VII The procedure of Example III is repeated with good results using a nickel plate and a gum arabic-ammonium dichromate photoresist, with 2 for 7 minutes each in the four directions, 5 for 15 minutes each in the four directions, and repeated at 5 for 15 minutes in the third cycle.

Example VIII The procedure of Example III is repeated with good results using a plate of chromium.

Example IX The procedure of Example VI is repeated with similar results using as a mordant in place of the FeCl a 23% aqueous solution of ammonium persulfate at 130 F. with exposure periods of 6 and 12 minutes in place of the 5 and 10 minute periods.

The type of mordant and the type of developer to be used will be determined by the type of resist or protective coating. Obviously, the developer must be one which will soften and remove the unhardened coating while at the same time not damaging or removing the hard resist. Manufacturers and suppliers of commercial resist materials generally recommend and supply a developer that can be used with the particular resist materials. Ferric chloride solution (3844 Baum, preferably 4042) generally is preferred as a mordant for use in the practice of this invention. However, other mordants which will attack the particular metal also can be used, provided the resist is inactive toward that particular mordant and sufficiently Well adhered to the metal to prevent undercutting. Other mordants which can be used with particular resist materials inactive to the mordant are: nitric acid, acetic acid, solutions of hydrogen chloride in alcohol or glycerine, ferric chloride solutions containing minor amounts of hydrogen chloride, aqueous ammonium sulfate, etc.

The total period of exposure will depend on the metal being used and the depth desired in the bulbular opening, as well as the particular mordant being used. Obviously, a slower reacting mordant, a less reactive metal, and deeper openings will each require longer exposure periods. With stainless steel, such as 430 stainless steel, 3. total exposure time of about 4 hours gives a depth of about 0.017 inch using an FeCl mordant of about 41 Baum at F. A total period of about 30 minutes gives a depth of about 0.006 inch. In comparison, carbon steel reacts between 2 and 4 times as fast and, therefore, the exposure periods are reduced correspondingly. Roughly, aluminum reacts about 3-5 times as fast as carbon steel, copper about 2-3 times as fast, nickel about 1-2 times as fast, and chromium about one-half as fast as carbon steel, and the exposure periods are adjusted accordingly.

Plates having a plurality of openings as described herein are useful in the manufacture of a particular type of nuclear battery using minute pellets or powder of radioactive material. A pellet or powder is inserted in each of the bulbular openings. During the long radioactive life of these materials, heat is generated and is transmitted through the metal. By proper spacing of these bulbular openings at appropriate distances from each other and spaced equally distant from adjacent openings, the heat generation can be distributed uniformly. A second plate having similar openings with identical arrangement, spacing, and dimensions, so that the various openings in one plate will register with openings in the other plate can be brought together face to face and the plates fastened together so that the openings are thereby closed. It is also possible to seal off the various openings by fastening a blank plate, one in which there are no openings, on to such a plate as described above, so that the openings are sealed. The heat distributed in the plate by the constant emission of radioactivity from the powder or pellets in the bulbular openings can be used to generate electricity by the thermocouple principle or to heat water which is allowed to flow on the other side of such a plate, or grooves actually can be designed in certain sections on the same side of the plate so as to pass between areas having such bulbular openings therein and thereby accumulate heat generated by the radioactive material. Steam generated by the accumulated heat from such plates can be used to turn a turbine and thereby generate electric power. For such purposes, openings of accurate uniform dimension and spacing as well as smoothness of surface and freedom from craters are desired.

While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course, be apparent that other modifica tions may be made within the spirit and scope of this invention and it is not intended to limit the invention to the exact details shown above except insofar as they are defined in the following claims.

I claim:

1. A process for the production of a plurality of minute bulbular openings of substantially uniform dimensions in a metal selected from the class consisting of iron, copper,

aluminum, nickel, and chromium, and alloys containing predominantly at least one of said metals, comprising the steps of applying a protective coating to the surface of said metal which is not to be etched and of exposing an unprotected area of the surface of said metal having a diameter in the range of about 0.006 to about 0.030 inch, to a mordant solution while said surface is submerged below the surface of said mordant solution and at an angle of 2-10 degrees to the surface of said solution, the area of said metal surface which is not to be exposed being protected from the reaction of the mordant solution by a coating which is resistant to the action of the mordant solution, and having a stream of air bubbles having an average diameter no greater than about /2. inch pass over said exposed area during said exposure, and thereafer removing said protective coating, said angle being maintained alternately in two directions approximately 180 from each other using opposite edges of the plate as the base of said angle.

2. The process of claim 1 in which said bubbles have an average diameter no greater than about /1 inch.

3. The process of claim 1 in which a plurality of such openings are produced simultaneously in said plate.

4. The process of claim 1 in which the sequence of said exposures is repeated using a larger angle in the second sequence.

References Cited UNITED STATES PATENTS 1,313,233 8/1919 Grass 156-5 2,881,059 4/1959 Spencer 15618 FOREIGN PATENTS 507,854 6/1939 Great Britain.

JACOB H. STEINBERG, Primary Examiner. 

1. A PROCESS FOR THE PRODUCTION OF A PLURALITY OF MINUTE BULBULAR OPENING OF SUBSTANTIALLY UNIFORM DIMENSIONS IN A METAL SELECTED FROM THE CLASS CONSISTING OF IRON, COPPER, ALUMINUM, NICKEL, AND CHROMIUM, AND ALLOYS CONTAINING PREDOMINANTLY AT LEAST ONE OF SAID METALS, COMPRISING THE STEPS OF APPLYING A PROTECTIVE COATING TO THE SURFACE OF SAID METAL WHICH IS NOT TO BE ETHCED AND OF EXPOSING AN UNPROTECTED AREA OF THE SURFACE OF SAID METAL HAVING A DIAMETER IN THE RANGE OF ABOUT 0.006 TO ABOUT 0.030 INCH, TO A MORDANT SOLUTION WHILE SAID SURFACE IS SUBMERGED BELOW THE SURFACE FO SAID MORDANT SOLUTION AND AT AN ANGLE OF 2-10 DEGREES TO THE SURFACE OF SAID SOLUTION, THE AREA OF SAID METAL SURFACE WHICH IS NOT TO BE EXPOSED BEING PROTECTED FROM THE REACTION OF THE MORDANT SOLUTION BY A COATING WHICH IS RESISTANT TO THE ACTION OF THE MORDANT SOLUTION, AND HAVING A STREAM OF AIR BUBBLES HAVING AN AVERAGE DIAMETER NO GREATER THAN ABOUT 1/2 INCH PASS OVER SAID EXPOSED AREA DURING SAID EXPOSURE, AND THEREAFTER REMOVING SAID PROTECTIVE COATING, SAID ANGLE BEING MAINTAINED ALTERNATELY IN TWO DIRECTIONS APPROXIMATELY 180* FROM EACH OTHER USING OPPOSITE EDGES OF THE PLATE AS THE BASE OF SAID ANGLE. 